Hard to see in your photo, but it looks like the hammer nose bushing may, in fact, be below the level of the breechface as your gunsmith indicated. If it is too low, the primers can back out due to lack of support.
If you have any of the brass you were shooting when the incident occurred, you can have a look at the primer area to see if there is damage or drag marks.
If the bushing is damaged, you will need to send it back to have the bushing replaced/repaired. This particular repair takes special tools and repair parts.

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Ret. LE, FA Instr, S&W Armorer